When driving brushless DC motors by way of pulse width modulation (PWM), the width of the voltage pulses applied to the phases of the motor is used as control parameter to control the current in the motor phases. The width of the applied voltages to the motor coils windings is thereby often applied such that the current in the motor phases has a sinusoidal or similar shape versus time.
The applied PWM signals on each of the motor phases may be out of phase to each other, for a 3 phase motor they are usually 120 degrees out of phase to each other. The used waveform profiles to generate the PWM duty cycle (the percentage of the PWM on time vs. the PWM period duration) may have different characteristics, but typically lead to sinusoidal current waveforms.
The amplitude of the motor currents determines the motor output speed and torque. The amplitude of the current is manipulated by applying a duty cycle scaling. This duty cycle is calculated by multiplying the phase related (e.g. sinusoidal) value of a waveform at a given rotor angle with a scaling factor. The motor output speed/torque follows the scaling factor.
When the scaling factor becomes too small, then the applied PWM duty cycle and the resulting current are not proportional anymore, because the time to switch on and off the current (e.g. via MOS transistor) becomes non-neglectable compared to the duty cycle itself. This leads to a distortion of the motor torque and consequently to a less efficient motor operation and audible noise.
Another problem is that the measurement of motor operating parameters like for example motor current and rotor induced voltages (the so called BEMF voltage) that should be measured ideally during the PWM on time is inaccurate, because the PWM on period is too short and ringing caused by the PWM switching process itself disturbs accurate measurements. This can lead to inefficient motor control and torque ripples.
Therefore, in view of the control difficulties at low speed/torque, there is still room for improvement in controlling motors which are driven by way of pulse width modulation.